BUILDING A WEB APPLICATION
Create a Web Controller
In Spring’s approach to building web sites, HTTP requests are handled by a controller. You can easily identify the controller by the @Controller annotation. In the following example, GreetingController handles GET requests for /greeting by returning the name of a View (in this case, greeting). A View is responsible for rendering the HTML content. The following listing (from src/main/java/com/example/servingwebcontent/GreetingController.java) shows the controller:
COPYpackage com.example.servingwebcontent;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
@Controller
public class GreetingController {
@GetMapping("/greeting")
public String greeting(@RequestParam(name="name", required=false, defaultValue="World") String name, Model model) {
model.addAttribute("name", name);
return "greeting";
}
}
This controller is concise and simple, but there is plenty going on. We break it down step by step.
The @GetMapping annotation ensures that HTTP GET requests to /greeting are mapped to the greeting() method.
@RequestParam binds the value of the query string parameter name into the name parameter of the greeting() method. This query string parameter is not required. If it is absent in the request, the defaultValue of World is used. The value of the name parameter is added to a Model object, ultimately making it accessible to the view template.
The implementation of the method body relies on a view technology (in this case, Thymeleaf) to perform server-side rendering of the HTML. Thymeleaf parses the greeting.html template and evaluates the th:text expression to render the value of the ${name} parameter that was set in the controller.The following listing (from src/main/resources/templates/greeting.html) shows the greeting.html template:
COPY<!DOCTYPE HTML>
<html xmlns:th="https://www.thymeleaf.org">
<head>
<title>Getting Started: Serving Web Content</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>
<body>
<p th:text="'Hello, ' + ${name} + '!'" />
</body>
</html>
Make sure you have Thymeleaf on your classpath (artifact co-ordinates: org.springframework.boot:spring-boot-starter-thymeleaf). It is already there in the "initial" and "complete" samples in Github.
Spring Boot Devtools
A common feature of developing web applications is coding a change, restarting your application, and refreshing the browser to view the change. This entire process can eat up a lot of time. To speed up this refresh cycle, Spring Boot offers with a handy module known as spring-boot-devtools. Spring Boot Devtools:
- Enables hot swapping.
- Switches template engines to disable caching.
- Enables LiveReload to automatically refresh the browser.
- Other reasonable defaults based on development instead of production.
Run the Application
The Spring Initializr creates an application class for you. In this case, you need not further modify the class provided by the Spring Initializr. The following listing (from src/main/java/com/example/servingwebcontent/ServingWebContentApplication.java) shows the application class:
COPYpackage com.example.servingwebcontent;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class ServingWebContentApplication {
public static void main(String[] args) {
SpringApplication.run(ServingWebContentApplication.class, args);
}
}
@SpringBootApplication is a convenience annotation that adds all of the following:
- @Configuration: Tags the class as a source of bean definitions for the application context.
- @EnableAutoConfiguration: Tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings. For example, if spring-webmvc is on the classpath, this annotation flags the application as a web application and activates key behaviors, such as setting up a DispatcherServlet.
- @ComponentScan: Tells Spring to look for other components, configurations, and services in the com/example package, letting it find the controllers.
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The main() method uses Spring Boot’s SpringApplication.run() method to launch an application. Did you notice that there was not a single line of XML? There is no web.xml file, either. This web application is 100% pure Java and you did not have to deal with configuring any plumbing or infrastructure.
Build an executable JAR
You can run the application from the command line with Gradle or Maven. You can also build a single executable JAR file that contains all the necessary dependencies, classes, and resources and run that. Building an executable jar makes it easy to ship, version, and deploy the service as an application throughout the development lifecycle, across different environments, and so forth.
If you use Gradle, you can run the application by using ./gradlew bootRun. Alternatively, you can build the JAR file by using ./gradlew build and then run the JAR file, as follows:
java -jar build/libs/gs-serving-web-content-0.1.0.jar
If you use Maven, you can run the application by using ./mvnw spring-boot:run. Alternatively, you can build the JAR file with ./mvnw clean package and then run the JAR file, as follows:
java -jar target/gs-serving-web-content-0.1.0.jar
The steps described here create a runnable JAR. You can also build a classic WAR file.
Logging output is displayed. The application should be up and running within a few seconds.
Test the Application
Now that the web site is running, visit https://localhost:8080/greeting, where you should see “Hello, World!â€
Provide a name query string parameter by visiting https://localhost:8080/greeting?name=User. Notice how the message changes from “Hello, World!†to “Hello, User!â€:
This change demonstrates that the @RequestParam arrangement in GreetingController is working as expected. The name parameter has been given a default value of World, but it can be explicitly overridden through the query string.
Add a Home Page
Static resources, including HTML and JavaScript and CSS, can be served from your Spring Boot application by dropping them into the right place in the source code. By default, Spring Boot serves static content from resources in the classpath at /static (or /public). The index.html resource is special because, if it exists, it is used as a "`welcome page,"serving-web-content/ which means it is served up as the root resource (that is, at `https://localhost:8080/). As a result, you need to create the following file (which you can find in src/main/resources/static/index.html):
COPY<!DOCTYPE HTML>
<html>
<head>
<title>Getting Started: Serving Web Content</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>
<body>
<p>Get your greeting <a href="/greeting">here</a></p>
</body>
</html>
When you restart the application, you will see the HTML at https://localhost:8080/